It is known as to anodize aluminum or its alloy within an electrolytic solution, such as an aqueous solution, of nitric acid, sulphuric acid, or chromic acid, to form a corrosion resistant oxide film. Such alumite treatment is widely utilized in various fields, for example aircraft, automobile, marine vessels, optical instruments, instruments for the chemical industry, and even daily needs such as a pan and a teakettle.
However, an upper surface of the alumite film is generally porous. Therefore, in order to improve the corrosion resistance of the porous layer, it is required to perform one of various sealing treatments, e.g. to dip the product into boiling water.
Further, an alumite film is generally of a silver white color. Therefore, when a colored product such as a building material and daily needs, is desired, it is necessary to provide a coloring treatment in which a dye or a pigment is impregnated into the porous layer of the alumite film. Further, a process for forming a natural color anodic oxidation coating by electrolysis using an electrolyte containing sulphuric acid with sulfosalicylic acid added thereto is also known. However, any of the above described processes can color only a shallow area of the upper layer of the alumite film and thus the colored area is likely to subject to wear and discoloration, so that the alumite film is not necessarily sufficient durable because a deep portion under the shallow area remains porous.
It is an object of the present invention to eliminate the above-described disadvantages of the porior art and to provide a process for the surface treatment of aluminium or aluminium alloys, which is able to color various articles and does not use a toxic material, such as cyanogen, and can produce articles having excellent corrosion and abrasion resistance.